Origin and timing of siderite and calcite concretions in late Palaeogene non- to marginal-marine facies of the Te Kuiti Group, New Zealand

Abstract

The Late Eocene-earliest Miocene Te Kuiti Group represents an overall transgressive sequence of formations ranging upwards from non-marine coal measure facies through marginal-marine low-energy shoreline siliciclastic sediments to fully-marine mixed siliciclastic-carbonate deposits. Concretionary structures are common in the lowest formations of the group, being dominantly sideritic in the non-marine to brackish sediment facies (Waikato Coal Measures and Glen Afton Claystone) and calcitic in the more marine-influenced sediments (Rotowaro Siltstone). The geochemistry of the carbonate cements within these early diagenetic concretions records the change from carbonate precipitation from purely meteoric fluids to precipitation from marine fluids, and a shifting source of carbonate carbon across this transition.Siderite precipitation in the Waikato Coal Measures concretions and hardpans was initiated and often largely completed in the methanogenic zone, within the upper 20 m of the sediment pile over a period possibly lasting 100–300 ka. Precipitation continued at a reduced rate to burial depths of 300 m or more when septarian cracking occurred in some siderite concretions and was healed by calcite vein cements that probably derived carbonate from the oxidation of methane ascending from the maturing coal seams directly below.Siderite precipitation in the concretions and hardpans of the overlying brackish-marine Glen Afton Claystone was completed in the topmost few metres of sediment, possibly a few 10 ka. Stable-isotope values near zero support a marine carbonate carbon source, suggested to be from remobilised shell material as evidenced by the common occurrence of shell casts in the formation and the high degree of Mg and Ca substitution in the siderites.The overlying marginal-marine to low-energy marine Rotowaro Siltstone contains calcite concretions precipitated from pore waters enriched in bicarbonate derived from the sulphate-reduction zone, largely within the upper 8 m of sediment, and possibly growing over a few 100 ka. Concretion development continued at a reduced rate to burial depths of several tens of metres and in some cases septarian cracks developed during the later stages of concretion growth. The precipitation of septarian vein calcites with more depleted δ18O values and more enriched δ13C signatures than central concretions cements, may be due to increasing burial temperatures, and the progressive introduction of carbonate sourced from shell dissolution.